Null balance indicating and control apparatus and phase sensitive pulse responsive circuits for use therein

ABSTRACT

In apparatus and circuits especially suitable in instrumentation for indication and control of nuclear power plant coolant systems, a bridge contains in a sensing arm a resistance thermometer or a differential transformer. The output of the bridge is supplied to a servo amplifier which feeds a servo motor which is operatively connected to a bridge arm to utilize an error signal to rebalance the bridge and reduce the error signal to zero, and at the same time supply an input to a counting or indicating device. Unique circuits provide control set points, isolation being provided between the indication and control functions. New and improved phase sensitive monostable multivibrator transistor circuits are employed.

O Unlted States Patent 1191 1111 3,783,356 Lide, III et al. Jan. 1, 1974NULL BALANCE INDICATING AND 2,911,588 11/1959 Wetherhold, 323/75 8 xCONTROL APPARATUS AND PHASE 2,921,262 1/1960 Jaffe 323/75 B X 3,076,129l/l963 Bckemuehl.. 323/75 E UX SENSITIVE PULSE RESPONSIVE CIRCUITS3,165,681 1/l965 Pinckaers 3l8/28 FOR USE THEREIN 3,250,960 /1966Walbridge..... 323 75 E x 75 Inventors: Basil M. Lide, n Pittsburgh;Harry 3,378,024 4/1968 Kruik 3l8/28 X Kowalcheck, West Newton, both ofPa. Primary ExaminerBenjamin Dobeck [73] Assigneez Westinghouse ElectricCorporafion, Attorney-A. T. Stratton, C. L. McHale and M. l. HullPittsburgh, Pa.

22 Filed: May 6, 1965 [57] ABSTRACT 21 Appl. No.: 453,692

US. Cl 318/18, 318/28, 323/75 In apparatus and circuits especiallysuitable in instrumentation for indication and control of nuclear powerplant coolant systems, a bridge contains in a sensing [52] arm aresistance thermometer or a differential trans- [51 Int. Cl. G05b 11/14fol-men The output of the bridge is Supplied to a Servo [58] Field ofSearch 323/123, 75 B, 75 E, amplifier Which feeds a Servo motor which isOpera 323/203705 20360; 318/208), tively connected to a bridge arm toutilize an error sig- A nal to rebalance the bridge and reduce the errorsignal 2 to zero, and at the same time supply an input to a [56]References cued counting or indicating device. Unique circuits provideUNITED STATES PATENTS control set points, isolation being providedbetween 3,171,075 2 1965 Kirk 318/341 the indication and Controlfuilctions- New and 3,105,180 9/1963 Burnett... 318/341 X proved phasesensitive monostable multivibrator tran- 3,004,199 /1961 Sakson 318/28sistor circuits are employed. 2,659,850 ll/l953 Phillips et al. 1318/341 X 2,246,575 6/1941 Coleman 323/75 B UX Claims, 16 DrawingFigures 68 70 f 69 I ,4 GEAR 46 48 1 65 TRAIN I 3 7B 1 1 RESISTANCE 66e7 THERMOMETER BR'DGE s AM Ei iER ti i 7 POWER 1 SUPPLY 49 1 i 1 i 54 15s 1 58 59 60 6| amuse A394 PHASE FLIP POWER AMPLIFIER SENSOR FLOPAMPLlFlER U minnow new sum 2 or 4 UTIUZATION AND CONTROL AMPLIFIER PHASESENSOR FLIP FLOP AND UTILIZATION I INVENTORS Basil M. Lide 5 HorryKowolcheck ATTO NEY NULL BALANCE INDICATING AND CONTROL APPARATUS ANDPHASE SENSITIVE PULSE RESPONSIVE CIRCUITS FOR USE THEREIN This inventionrelates to improvements in indicating and control apparatus, and moreparticularly to an improved circuit employing a single transducerproviding both indication and control functions, the circuit supplying anull balance servo type of indication, the circuit including a phasesensitive pulse responsive control amplifier feature to supply a controlfunction which is substantiallyindependent of the indication function. g

The apparatus and circuit of the instant invention are especiallysuitable in instrumentation for indication and control of nuclear powerplant coolant systems, although the use of the apparatus and circuits isnot limited thereto. Prior art indication and control circuits andapparatus employed with nuclear power plant coolant systems, generallyspeaking, make use of linear open-loop circuits to provide an indicationof a plant parameter. Output signals from linear devices are also usedto operate bistable amplifiers for control functions, such as safetyshut-down. These prior art devices, circuits and systems arecharacterized by a number of disadvantages: all gains must be stabilizedto a high degree; supply voltages must be well regulated; compensationmust be provided for supply voltage frequency changes; compensation mustbe provided for ambient temperatue changes; and others.

Accuracy requirements under the wide variation of equipment ambientconditions have made refinement of the open loop systems impractical.These ambient conditions may be as follows: line voltages, 115 i 10percent nominal; frequency, 60 cycles 1*; 5 percent; voltage transients,i 30 percent from nominal; and ambient temperature ranges, 25 to 65C.When it is recalled that the desired overall accuracy of the outputsignals is, for example, one-half of 1 percent on reactor coolanttemperature measurements, and 2 percent on reactor loop pressuremeasurements under the worst combination of the above conditions, it isapparent that these accuracy requirements under the wide variation inconditions could be met only by the most expensive and refinedequipment.

The apparatus of the instant invention overcomes I these and otherdisadvantages of the prior art. In summary, our apparatus in one typicalapplication includes but is not limited to a transducer, which may be aresistance thermometer or a differential transformer, connected in abridge circuit which contains a rheostat or potentiometer having anadjustable resistance value, such as may be obtained by use of an armwith an adjustable setting; the bridge may be maintained in balance byadjusting the position 'of the arm. The bridge is excited by analternating current of selected frequency,.usually the supply linefrequency for convenience. While D.C. voltages could be used for thebridge supply, the advantage of drift-free amplification would not beeasily attained as with A.C. amplifiers, as will be seen later. In theindication portion of our apparatus, an output of the bridge circuit issupplied to a servo amplifier, which feeds a servo motor, which isoperatively connected to a bridge arm to utilize an error signal torebalance the bridge and reduce the error signal to zero, and at thesame time supply an input to a counting or indicating device. In usualapplications of null balance indicators such as servo operated slidewire recorders, for example, when the need arises for on-off controlpoints these are derived by mechanical means such as switches and camsoperated from the slide wire drive shaft or from circuits operating fromauxiliary tandem slide wires. Such systems have the followingdisadvantages.

l. The control function reliability depends on me chanical means, suchas moving slide wires and gears, and is relegated to a position ofsecondary importance in respect to the indicating function. Such thingsas chart paper, pens, etc. contribute to unreliability of such devicesif used for control. 2. Response time of control functions depend on theslow moving servo systems.

In summary, the circuits of our invention which provide for control setpoints employ the sensing element portion of the bridge with a separatemanually adjustable balance arm for signal to the control pointcircuits. By proper choice of input impedances nearly complete isolationis provided between the indication and control functions. To obtainaccurate trip point of the control circuits the control arin of thebridge is operated close to bridge balance by use of high gainamplification of control bridge error and utilizing phase change of theoutput of the amplifier to detect a. change through balance of thebridge sensing element. Thus, the higher the amplifier gain the moreaccurate the trip point since the bridge output approaches zero at tripas the amplifier approaches infinite gain. Dependence on gain stabilityof the amplifier and the bridge supply is eliminated since the tripsignal occurs at bridge null with almost zero into the amplifier.

The output signal which appears when the bridge goes through null, asdetected by the phase sensing am plifier, is supplied to a monostablemultivibrator circuit and causes the monostable circuit to change from afirst condition or state to a second condition or state.

Accordingly, a primary object of our invention is to provide new andimproved null-balance servo control apparatus.

Another object is to provide a new and improved phase sensitive pulseresponsive circuit for use in control applications.

An additional object is to provide a new and improved null balance servoindication and control system especially suitable for instrumentationand control of nuclear power coolant systems. i

A further object is to provide a new and improved phase sensitivemonostable multivibrator transistor circuit.

Still another object is to provide apparatus for controlling the coolantsystem of a nuclear power plant in which control and indicationfunctions, while necessitating only a single transducer, arenevertheless substantially independent of each other.

Still a further object is to provide new and improved control andindicating apparatus the accuracy of which will not be substantiallyinfluenced by wide variations in ambient conditions and variations inpower input.

These and other objects will become more clearly apparent after a studyof the following specification, when read in connection with theaccompanying drawings, in which:

FIGS. 1A and 1B show linear open loop systems according to the priorart, the system of FIG. lA employing a resistance thermometer bridge,and the system of FIG. 1B employing a differential transformer;

FIG. 2 is a schematic electrical circuit diagram of a resistancethermometer bridge and a phase sensitive monostable multivibratortransistor circuit as connected in the control portion of our invention;

FIG. 3 is a view of the circuit of FIG. 2 partially in block form asmodified to employ a differential transformer as the transducing elementof the bridge;

FIG. 4 is a schematic electrical circuit diagram of a phase sensitivemonostable circuit according to another embodiment of our invention;

FIG. 5 is a block diagram of the apparatus and circuit of our inventionshowing both the indication and con trol portions of the apparatus;

FIG. 6 is a fragmentary view of a portion of the circuit of ourinvention according to one embodiment thereof;

FIG. 7 is a fragmentary view similar to FIG. 6 in which a differentialtransformer is utilized as the transducing element;

FIGS. 8A, 8B and 8C are a series ,of graphs illustrating the operationof the apparatus of FIG. 2 and FIG.

FIG. 9 'is a fragmentary circuit portion showing a bridge arrangementsuitable for use in the apparatus of FIG. 2 when employed in thecomplete system of FIG. 5;

FIG. 10 is an additional fragmentary bridge portion of a circuitsuitable for use in the circuit of FIG. 4 when employed in thearrangement of FIG. 5; and

FIG. 11 is a fragmentary circuit diagram of a modified portion of, thecircuit of FIG. 2 according to an additional embodiment of theinvention.

Referring now to the drawings, in which like reference numerals are usedthroughout to designate like parts, for a more detailed understanding ofthe invention, and in particular to FIG. IA thereof which shows a priorart arrangement, a resistance thermometer bridge 21, which acts as atransducer for providing an electrical signal which varies in accordancewith variations in some quantity, is shown supplying its output to adirect current amplifier 22, which feeds a bistable or astable amplifier23, which controls a controlled device, not shown for convenience ofillustration, the direct current amplifier also providing an indicationat 24 of the value of the input quantity. It is noted that regulatedpower supplies 25, 26 and 27 are required for the circuit devicesincluding the bridge 21, the direct current amplifier 22 and thebistable or astable amplifier 23 respectively, and that means forambient temperature compensation is provided for each of these, theseambient temperature compensating means being shown in block form at 28,29 and 30 respectively. Those skilled in the art will readily understandthe additional complexity and expense necessitated by providingregulated power, and temperature compensation. The nature of thisarrangement is shown in FIGS. 1C

and 1D. In FIG. 1C, the output of the D.C. amplifier 22 is plotted as afunction of input, with the desired trip point indicated by the point X.The trip point of the bistable or astable amplifier 23 is indicated inFIG. 1D, where output of the bistable or astable amplifier is shown as afunction of input, the trip point being indicated at point 2.

In FIG. 1B a prior art arrangement is shown employing a differentialtransformer as the transducer, the differential transformer 31 supplyingits output by way of frequency compensating means 36 to a bridgerectifier 32, which supplies an output to direct current amplifier 33,which supplies an output to indicator 34 and bistable amplifier 35. Itis noted that the differential transformer 31 must be supplied withregulated alternating current power from source 37, and must haveambient temperature compensation by means 38. In addition, the D.C.amplifier 33 must have ambient temperature compensation by means 39 andregulated power from source 40, and the bistable amplifier 35 must haveambient temperature compensation by suitable means 41 and regulatedpower from source 42. Curves similar to those of FIGS. 1C and 1D wouldrepresent the outputs of the D.C. amplifier 33 and the bistableamplifier 35 respectively.

Particular reference is made now to FIG. 5, showing in block form acircuit and apparatus according to one embodiment of our invention. Aresistance thermometer, which has a resistance value which varies withvariations in the temperature thereof, shown in block form at 46, isconnected by lead means 47 to two bridge circuits shown in block form at48 and 49. As will be seen hereinafter, these bridges may have a commonbranch containing the transducer. Bridge 49 has alternating currentsupplied thereto by lead means 50 from power supply 51 which providesboth alternating current and direct current as needed, the bridge 49supplying its output by lead means 52 to an alternating currentamplifier 53, which supplies an output by lead means 54 to a phasesensitive circuit 55 which also has alternating current of the samefrequency supplied thereto from power supply 51. When the input to thephase sensor circuit 55 is of a certain polarity and phase relationshipto the alternating current, and attains a certain amplitude, the outputof the phase sensor as developed on lead means 56 causes the flip-flop57 to change from its steady state to another quasi-stable state andsupply or cut off an output to a power amplifier, the output of theflip-flop 57 being supplied to the power amplifier 59 by lead means 58.Power amplifier 59 may be a silicon controlled rectifier, and associatedcircuitry. The output of the power amplifier 59 is supplied by leadmeans 60 to the control device 61 which may be, for example, a valvecontrolling the temperature of the chamber or device where resistancethermometer 46 is located. Power amplifier 59 may be omitted if desired.

The aforementioned resistance thermometer 46 is also connected to anadditional bridge circuit 48,'which may be similar to the bridge circuitof FIG. 6. The output of the bridge 48 is supplied by lead means 78 to aservo amplifier 65 which supplies an output by lead means 66 to a servomotor shown in block form at 67. The servo motor is mechanicallyconnected to a gear train device 68, which is operatively connected to acounting or indicating device 69, and which is also connected bymechanical coupling 70 to the arm 71 of a potentiometer in the bridgecircuit, FIG. 6, to which particular reference is made. It is seen thatalternating current from source 51 is supplied by leads 73 and 74 acrossa bridge consisting of resistor 75 and resistance thermometer 46 in onebranch thereof, and resistor 76 and variable resistor 77' in the otherbranch thereof. The junction between resistor 75 and thermometer 46 isconnected by way of lead 78 to one input terminal of the servo amplifier65, whereas the junction between resistor 76 and variable resistor 77 isconnected by way of lead 78' to the other input terminal of the servoamplifier 65. The arm 71, which may have the setting thereof on resistor77 varied by mechanical linkage 70, provides for varying the totalresistance of resistor 77.

It will be seen from a study of the circuit of FIG. 6 that the nullbalance arrangement renders the circuit independent of certain varyingparameters and varying ambient and power supply conditionsaforementioned. In FIG. 6, let resistor 75 have a value R and letresistor 76 have a value K R. The resistance thermometer 46 hasa valueR, and a voltage E is developed between lead 74 and lead 78. Theresistor 77 has a value K,R, and a voltage E is developed between lead78 and lead-74. The value of E is a function of temperature; when E Ethe input to the amplifier, that is E E is zero, and the counter 69reads temperature. When temperature increases, R, increases, Eincreases, and the difference E E is applied to the amplifier. The motorruns the servo potentiometer until E E and then stops. The counter readsthe new temperature. At balance it will be noted that line voltagevariations affect E and E identically, and E E remains zero regardlessof line voltage fluctuations.

Particular reference is made now to FIG. 7, in which an embodiment ofthe invention according to FIG. 5 but employing a differentialtransformer as the transducer of the bridge, is shown. The differentialtransformer is generally designated 85 and has a primary 86, a movableslug 87 connected to mechanical linkage 88, a pair of secondaries 89 and90, and a junction between secondaries 93. It will be readily understoodthat the slug position can be proportional to pressure, or the levelfrom a Bourdon tube, or bellows, the slug being made of a suitablematerial, for example iron, and varying the coupling between the primaryand the two secondaries in accordance with changes in the position ofthe slug. It is seen that potentiometer 94 is connected between leads 91and 92, which are connected to the outer terminals of secondaries 89 and90 respectively. In analyzing the circuit of FIG. 7, let E, be thevoltage across secondary 90, E be the voltage across secondary 89, and Ethe voltage between lead 96 connected to potentiometer arm 95, and lead92. At balance conditions, that is, no error and no input to theamplifier, the shaft counter reads the slug position, E, E where E E isa constant as determined beforehand by the nature of the transformerdesign. If the slug moves down, for example, on pressure increases, Eincreases, E decreases and E E equals a constant. Since E now does notequal E the difference appears as an input to the servo amplifier.Output from the amplifier runs the motor to raise the arm 95 of theservo potentiometer until E E,, and the motor stops. The counter nowreads the new pressure. At balance, changes in voltage or frequency, andvariations in the value of primary resistance all affect E and Eidentically, and the input remains E, E 0. Accordingly, the reading ofthe counter does not change with fluctuations in line voltage, changesin ambient temperature, or changes in the resistance of the primary.

Accordingly, it is seen that the circuit arrangement of FIG. 5, whenused either with a resistance thermometer or with a differentialtransformer, provides for an indication which is substantiallyindependent of variations utilized for control purposes, the phasesensor circuit, and the monostable multivibrator circuit are shown inschematic electrical circuit diagram. An alternating currenttransformer, which it will be understood may be included in the powersupply 51 of FIG. 5, has a primary 101 energized from any suitablealternating current source, not shown, a core 102 and secondaries 103,104 and 105. The output of secondary 103 is applied by way of leads 106and 107 across the two branches of a bridge arrangement, one of thesebranches including potentiometer 108 having movable contact or arm 109,and resistor 110. As employed herein the word branch includes two armsof a bridge, the arms being connected in series across the source. Theother branch of the bridge connected across leads 106 and 107 includesin series the resistance thermometer 46, lead 111 and resistor 112. Theadjustable arm 109 of potentiometer 108 provides a means of adjustingthe trip point, arm 109 being connected by way of lead 113, capacitor114, resistor 115 and lead 116 to the base 117 of a transistor 118. Lead116 is connected by way'of capacitor 119 to lead 111, and emitter 120 isconnected to lead 111.

The aforementioned secondary 105 has the terminals thereof connected toa full wave bridge rectifier generally designated 121, the negativeterminal of the bridge rectifier being connected to lead 111, thepositive terminal of the bridge rectifier being connected by way ofresistor 122, lead 123 and capacitor 124 to the aforementioned negativelead 1 1 1. It is accordingly seen that a positive potential isdeveloped between lead 123 and the aforementioned lead 111 which forms acommon negative return. Lead 123 is connected by way of resistor 125 tothe aforementioned lead 116 and base 117. Lead 123 is also connected byway of resistor 126 and lead 127 to the collector 128 of theaforementioned transistor 118.

The transistor 1 18 serves as a straight alternating current amplifierfor amplifying the signal applied between the base and emitter thereof.Resistor 115 and capacitor 119 provide, if desired, phase correction,and resistor 1 15 may be made variable if desired. The aforementionedcollector 128 is connected by way of capacitor 130 and lead 131 to thebase 132 of an additional transistor 133 having emitter 134 thereofconnected to lead 1 11 and having a collector 135. The transistor 133provides phase sensing in a manner which will be more clearly apparenthereinafter.

The aforementioned secondary 104 has one terminal thereof connected tolead 111 and the other terminal thereof connected by way of capacitor138, lead 139, rectifier 140, lead 141, resistor 142 and lead 143 to theaforementioned collector 135. Resistor 144 is connected from lead 111 tolead 139, whereas resistor 145 is connected from lead 111 to lead 141.

The aforementioned lead 123 is connected by way of resistor 146, lead147 and'resistor 148 to the aforementioned lead 131 and base 132. Lead147 is connected by way of rectifier 150 and lead 151 to the arm 152 ofa rheostat 153 having one end thereof connected by way of lead 154 andresistor 155 to the aforementioned lead 123. Lead 151 has capacitor 136connected there from to ground 205.

The aforementioned collector 135 and lead 143 are connected by way ofresistor 160, rectifier 161 and lead 162 to the base 163 of a transistor164 having an emitter 165 connected to lead 111. The aforementioned lead162 and base 163 are connected by way of resistor 166, lead 167,capacitor 168, lead 169 and resistor 170 to the aformentioned lead 123,which as aforementioned, is at a positive potential with respect to lead1 1 1. The aforementioned transistor 164 has a collector 172 connectedto the aforementioned lead 154. Collector 172 and lead 154 are connectedby way of resistor 174 to the base 175 of a transistor 176. The emitter177 of transistor 176 is connected to the aforementioned lead 111, whilethe collector 178 of the transistor 176 is connected to theaforementioned lead 169, thence to output terminal 179, and thence toutilization and control device 188, terminal 181, and lead 111, Device188 may control the temperature at 46. A rectifier 180 is connectedbetween the aforementioned lead 167 and lead 111 for purposes to becomehereinafter more clearly apparent.

In the circuit of FIG. 2, bridge output is developed between leads 113and 111, and after some slight phase shift, if desired, at 115-119, isamplified by transistor 118. It is noted that transistor 1 18 is an NPNtransistor; the positive potential from lead 123 by way of resistor 125applied to the base 117 forward biases the emitter 120.

Transistor 133 is a phase sensor. A pulsating direct current potential,obtained by half wave rectification of an alternating current potentialof the same frequency as that applied to the bridge, is applied to thecollector 135. Base 132 is normally biased slightly beyond saturationfrom lead 123 by way of resistor 146, lead 147, and resistor 148.Accordingly, it will be seen that positive alternations of the signaloccurring on base 132 simultaneously with the application of a DC. pulseto the collector cause no substantial output since the transistor isalready at saturation. On the other hand, negative alternations of thesignal on the base occurring simultaneously with the DC. pulses of halfwave rectified AC. on the collector produce an output, since during thenegative alternations, the base is not at saturation bias.

Transistors 164 and 176 comprise a multivibrator or flip-flop circuit.Rectifier 161 limits the current path in this portion of the circuit,while rectifier 180 provides a discharge path for capacitor 168 so thatthe circuit may readily change in either sense between states.

The circuit of potentiometer 153 and rectifier 150 is an adjustablemeans for changing the bias point of sensor transistor 133. Whentransistor 164 becomes conductive lead 154 falls in potential below thatof lead 147; in accordance with the polarity of rectifier 150, currentflows, increasing the voltage drop across resistor 146 and changing thebias on base 132 of transistor 133, that is, making the base lesspositive; the bias on transistor 133 decreases toward saturation, sothat thereafter the flip-flop will substantially remain on or in itsquasi-stable state when the input signal decreases below the originaltrip on value. Hence an adjustable means, potentiometer 153, is providedto control the trip off point.

In more fully understanding the operation of the circuit of FIG. 2, itshould be recalled that the purpose of this circuit is to cause themultivibrator comprising transistors 164 and 176 to switch from itsstable state to its quasi-stable state when a signal of at least apredetermined amplitude and of the proper phase appears between arm 109and lead 111. It will be understood that in one mode of operation wherearm 109 is initially adjusted for zero signal output, depending uponwhether the bridge is off balance in one sense or direction, or in theother direction, that is, whether the resistance value of 46 increasesor decreases from its initial balance value, the phase of the signalbetween lead 111 and 113 reverses or changes l.

Particular reference is made now to FIG. 8A, show ing signal conditionswhen the bridge is below balance in one sense or direction, where thecurve A of the group of waveforms represents the bridge output, curve Bindicates the amplifier output of amplifier 118, curve C the DC. pulseoutput of rectifier 140, and curve D the output of phase sensortransistor 133, or E, the collector voltage, all plotted on the sametime scale. FIG. 8B shows similar signals A, B, C and D at bridgebalance, and FIG. 8C shows the signals A", B", C", and D" when thebridge is above balance. When the bridge is above balance and a negativealternation of the output of amplifier 118 is applied to base 132coincident with a positive pulse applied to collector 135, an output D"is obtained from transistor 133. If the input A is 'of sufficientamplitude, the output D" exceeds the trip level, as shown, causing themultivibrator to change between states.

By suitable choice of component values, polarities and lead connections,it is arranged so that phase sensitive transistor 133 has an output whenthe bridge is unbalanced in the desired direction, that is, thedirection at which it is desired to utilize some result or indicationthereof and in at least a predetermined amount. The amount of requiredunbalance determines the trip point, which can also be adjusted withinlimits by arm 109. The control function occurs when the trip point isreached. The output of transistor 133 is applied by way of resistor andrectifier 161 to the base of transistor 164, the first transistor of aflip-flop circuit including also transistor 176. The operation of theflip-flop or monostable multivibrator circuit is to some extentunconventional. The base of the input transistor is coupled by way ofresistor 166 and capacitor 168 to the collector 178 of the secondtransistor 176, whereas the collector 172 of the first transistor 164 iscoupled by way of resistor 174 to the base 175 of the second transistor176, in accordance with conventional flip-flop circuitry. Accordingly,the voltage at terminal 179 connected to collector 178 varies betweentwo values depending upon which of the transistors 164 or 176 isconducting. While transistor 176 is conductive a very small voltage droptakes place across the transistor and accordingly terminal 179 is atsubstantially zero voltage with respect to terminal 181. When transistor176 becomes non-conductive terminal 179 rises to a relatively highvoltage with respect to lead 111 and terminal 181.

Before trip-on transistor 164 is not conducting. When it becomesconductive, it remains conductive until capacitor 168 is charged throughresistor 166, whereupon the circuit restores itself automatically to itsoriginal state. If the signal is still present at lead 131, the nextpulse or next negative alternation on base 132 causes the flip-flop toagain render transistor 164 conductive.

The apparatus or equipment to be controlled, or the element to becontrolled, shown in block form as a utilization and control device 188is operatively connected to the terminals 179 and 181. If desired, anemitter-follower transistor circuit may be connected between theseterminals having, for example, a relay shows the circuit of FIG. 2including the flip-flop amplifier connected'to a differentialtransformer to obtain a control signal therefrom. In FIG. 3 the primary191 has a slug 192 with mechanical linkage 193, and two secondaries 194and 195 connected by lead 196. Lead 196 is connected to one inputterminal of the apparatus shown in block form at 190, which may includean amplifier similar to 118, a phase sensor similar to 133, a flip-flopsimilar to 164-176, and a controlled device, and lead 196 may correspondto lead 111 of FIG. 2. Across the two secondaries is connected, inseries, the potentiometer 197 and resistor 198. The arm 199 of thepotentiometer is connected by lead 200 to the other input terminal ofthe apparatus 190, lead 200 corresponding to lead 113 of FIG. 2.

In the operation of the circuit of FIG. 3, as the slug 192 moves inresponse to variations in the value of some variable, which results inmovement being transmitted to the slug through the coupling 193, thesignals from the two secondaries 194 and l95 become unbalanced and-anunbalance signal is developed between leads 196 and 200 which issupplied to the input transistor corresponding to 118.

Particular reference is made now to FIG. 4 showing a modification of thecircuit of FIG. 2, in which an additional transistor is provided to takecontrol after the unbalance reaches at least a predetermined degree andthe multivibrator circuit has changed between states as a result of thatunbalance. In FIG. 4, an alternating current voltage is applied betweenthe leads 201 and 202; acrossthese two leads is connected one branch ofa bridge circuit including resistor 203, lead 204 connected to ground205, and resistance thermometer 206 which, it is understood, has aresistance value which varies in accordance with variations in aquantity, which variations are to be utilized for control purposes.Another branch of the bridge connected to leads 201 and 202 comprisesthe resistor 207, lead 208 and resistor 209. A third branch of thebridge is constituted by resistor 211, lead 2l2 and resistor 213.

Lead 208 is connected by way of capacitor 214 and lead 215 to the base216 of a transistor 217 having the emitter 218 thereof connected toground 205. The collector 220 is connected by way of lead'226, resistor221, lead 222 and resistor 223 to the positive terminal 224 of asuitable source of direct current potential, not shown, having thenegative terminal thereof connected to ground.

The aforementioned lead 215 is connected by way of resistor 225, lead226, capacitor 227, and lead 228 to the base 229 of a phase sensingtransistor 230 having the emitter 231 thereof connected to ground 205.The lead 228 and base 229 are connected by way of resistor 232 to thepositive terminal 233 of a source of direct current potential, notshown, having the other negative terminal thereof connected to ground205. The phase sensing transistor 230 has the collector 235 thereofconnected by way of lead 236, resistor 237, rectifier 238 and lead 239to one terminal of the secondary of a transformer which may correspondto the secondary 104 of FIG. 2, the other terminal of the secondarybeing connected to ground 205. Lead 239 may be identical with lead 139.The primary of the transformer whose secondary supplies an alternatingcurrent to lead 239 is preferably energized from leads 201 and 202. Inthis way a pulsating direct current is applied to the collector 235, andthis provides that the transistor 230 has an output only when apredetermined phase relationship exists between the voltage on thecollector and the alternating current signal applied to the basethereof.

The two transistors 241 and 242 comprise a monostable multivibrator orflip-flop circuit. The base 245 of transistor 241 is coupled by way oflead 244, resistor 243 and lead 236 to collector 235. The collector 252of transistor 242 is coupled by capacitor 254 and resistor 255 to thebase 245 of transistor 241, and the collector 247 of transistor 241 iscoupled by resistor 249 to the base 250 of transistor 242. Emitter 246of transistor 241 is connected to ground, and transistor 242 has emitter251 thereof connected 'to ground 205. As aforementioned, the collector252 of transistor 242 is connected by way of lead 253, capacitor 254 andresistor 255 to the aforementioned lead 244. Lead 253 is connected byway of resistor 256 to the positive terminal 257 of a suitable source ofdirect current potential, not shown, having the other negative terminalthereof connected to ground 205. Lead 253 and collector 252 are alsoconnected by way of resistor 260 to the base 261 of an emitter followertransistor 262 having the emitter 263 thereof connected by way of relaycoil 264 to ground 205. Relay 264 may control heater apparatus forvarying the temperature of transducer 206. Collector 265 is connected toa suitable source of direct current energizing potential, not shown,having the other terminal thereof operatively connected to ground.

An additional transistor 270 is provided, having an emitter 271, base272 and collector 273. The emitter 271 is connected to ground 205. Thebase 272 is connected by way of lead 274 and capacitor 275 to theaforementioned lead 212. Lead 274 is connected by way of resistor 276and lead 277 to the collector 273. Lead 277 is connected by way ofcapacitor 278 to lead 226. The aforementioned collector 273 oftransistor 270 is also connected by way of lead 277, resistor 280 andresistor 321 to the aforementioned lead 253. The junction betweenresistors 321 and 280 is connected by way of capacitor 322 to ground.

In understanding the operation of the apparatus of FIG. 4, it should beborne in mind that the circuit provides for a wide loop width and aflip-flop trip-off point adjustable over the entire range to the trip-onpoint. The term loop width as employed herein is used to designate themagnitude range over which the variable quantity may vary between thetrip-on point and the trip-off point. This is accomplished by utilizingthe multivibrator to control its own input signal. It will be noted thatwhile transistor 242 is conductive, which it normally is until a tripsignal causes transistor 241 to conduct and transistor 242 to becomenon-conductive, a very small voltage drop approaching zero appearsacross transistor 242 and accordingly substantially zero potential isapplied by way of lead 253 and resistor 280 to the collector 273 oftransistor 270 to provide no energizing potential thereto. On the otherhand, when the flip-flop circuit of transistors 241 and 242 renderstransistor 242 non-conductive, a substantial voltage from terminal 257appears on lead 253 providing an energizing potential to the transistor270. Transistor 270 is referred to herein as the off transistor, whereastransistor 217 is referred to as the on transistor.

Transistor 217 amplifies the output of the bridge portion comprisingresistors 203, 206, 207 and 209, and this output is amplified andapplied as a signal to the base 229 of transistor 230, which is normallybiased slightly beyond saturation. Transistor 230 has a pulsating directcurrent potential applied to the collector thereof and produces anoutput only when a predetermined desired phase relationship existsbetween the signal applied to the base 229 and the pulsed direct currentsignal applied to the collector 235.

Assume by way of example that with the bridge comprising resistors 203,206, 207 and 209 balanced, so that no signal is developed at base 229,and assuming further that in this condition transistor 242 is conductiveso that no substantial voltage appears on lead 253 and is applied totransistor 270, then when the trip-on point is reached, that is, whenthe voltage difference resulting from temperature changes on transducer206 is sufficiently great, the flip-flop circuit is caused to change toits other state, saturating transistor 241 and cutting off transistor242. This results in the application of an energizing potential to thecollector 273 of transistor 270 as well as supplying a biasing or gatingor energizing potential to the base 261 of the emitter followertransistor 262 resulting in the energizing of the relay coil 264.

Resistor 213 can be adjusted in value with respect to resistor 211 toprovide that until the desired trip-off point, the bridge may remainsufficiently unbalanced to provide a signal of sufficient amplitude totransistor 270, which signal is amplified and applied by way of couplingcapacitors 278 and 227 to the base 229 of the phase sensor transistor230, where, assuming that the desired phase relationship exists, theamplified signal is applied to the base of transistor 241 causing thecircuit again to substantially remain in its quasi-stable state, therebycausing transistor 241 to remain saturated, causing transistor 242 toremain cut off, causing the energizing potential to remain on collector273 of transistor 270, and causing the relay winding 264 to remainenergized until the trip-off point is reached.

It will be understood that transistor 241 may periodically returnmomentarily to a cut off state, but that within one cycle themultivibrator retrips, if the necessary signal is still present at base229.

It is seen then that by the addition of another bridge resistance pathin parallel with the branch of resistors 207 and 209, and the additionaltransistor 270, that a trip-off point of adjustable sensitivity isprovided to cause the relay winding 264 to be energized and thereafterdeenergized at any desired value of bridge unbal-. ance corresponding toany desired variation in the quantity which exercises control.

It should be understood that the term trip on refers to turning ontransistor 164, or transistor 241, depending on the circuit employed,and may result in turning off the heat which would result in increasedtemperature at transducer 46 or 206 or increased pressure and furthermovement of slug 87 or 192.

As an example of the operation of the circuit of FIG. 4, assume thatwhen relay 264 is energized it turns the heat off. Assume that theapparatus is at an initial temperature of 400, that it is desired toraise it to 500 and have the temperature stabilized between 500 to 490,a loop width of 10.

From previous calibrations, resistor 209 is adjusted to a value whichcauses that branch to balance, or produce zero output, at 500. Fromprevious calibrations, resistor 213 is adjusted to a value which causesthat branch to balance, or produce zero output, at 490. The heat isturned on. Transistor 241 is normally off and relay 264 normallydeenergized. As the temperature at 206 approaches but is below 500,there is a signal on lead 208 but it is the wrong phase. After 500 hasbeen reached and slightly passed, the signal on lead 208 becomes of theproper phase, tripping transistor 241 on, energizing transistor 270 andrelay 264 and turning the heat off. With 10 unbalance in that branch,there is now a large signal of the proper phase on lead 212, whichmaintains transistor 241 on or more precisely, keeps repeatedly turningit back on immediately every time it returns from its unstable orquasi-stable state to its normal stable state. As the temperature at 206gradually falls after the heat is turned off, the signal on lead 212diminishes in amplitude, and at balance, 490, suddenly reverses inphase, allowing transistor 241 to become off. This turns the heat backon, the temperature at 206 begins to increase, and the cycle isrepeated.

The circuit of FIG. 4 may be used with the differential transformer ofFIG. 3 by supplying in parallel with branch 197-198 another branchincluding resistor 21 1, lead 212 and resistor 213, with appropriateconnections being made.

Particular reference is made now to FIG. 9, which shows a modificationof the circuits of FIGS. 2 and 5, in which one bridge has a pair ofparalleled resistance paths in one branch, one path of the paralleledpaths having a potentiometer therein for supplying a signal to themonostable amplifier circuit, and the other of the paralleled resistancepaths having therein a potentiometer having an arm which is operativelyconnected to a servo motor for providing a null balance indication typeof circuit. In FIG. 9, the leads 291 and 292 have an alternating currentvoltage impressed thereacross, and may be connected to the power supply51 of FIG. 5, or may be connected to and supplied from the powertransformer of FIG. 2. One branch of the bridge includes a resistor 293connected by way of lead 294 to a resistance thermometer 295, theresistance value of which varies in accordance with variations in thetemperature. The lead 294 is connected to ground 205. In the otherbranch, one parallel resistance'path includes resistor 296 connected byway of potentiometer 297 to the opposite lead 292. The-arm 298 ofpotentiometer 297 is connected to lead 52 and supplies an input to atransistor amplifier. Lead 52 of FIG. 9 may correspond approximately tolead 113 of FIG. 2. The other paralleled resistance path includesresistor 301 and potentiometer 302 in series across leads 291 and 292.The arm 303 of potentiometer 302 is connected to lead 78, which maycorrespond to lead 78 of FIG. 5, it being understood that the otherinput terminal to the servo amplifier, not shown, is connected to ground205. The linkage 304 may correspond to the linkage of FIG. 5.

In order that errors in the indication device connected to mechanicallinkage 304 may be reduced to a minimum, it is desirable that the lead52 be connected to a high impedance load and this is contemplated, theload impedance being as high as convenient in view of the transistorcircuitry involved.

By adjusting the position of arm 298, the trip-on point of the amplifiercircuit of FIG. 2 may be adjusted, and this static trip may besubstantially independent of movement of the arm 303 in response torunning of the servo motor to affect a null balance and provide anindication accordingly. It will be understood that in the adjustment andcalibration of the servo and the indication device, account will betaken of the parallel circuit including resistors 296 and 297.

Particular reference is made now to FIG. 10, in which the circuits ofFIGS. 4 and 5 are combined, one resistance thermometer being employed ina bridge arrangement having three parallel circuits or three parallelresistance paths in one branch thereof. The additional path of theparallel paths of FIG. includes resistor 307 and potentiometer 308having arm 309 connected to lead 212. Accordingly, it is seen that oneresistance thermometer, in this case thermometer 295, is used in abridge arrangement which provides two signals to the monostablemultivibrator and amplifier of FIG. 4 to provide both a trip-on and atrip-off signal, and at the same time provides a signal to a servoamplifier, causing a servo motor to produce a null balance by anadditional potentiometer arm. It will be understood that in thecalibration of the circuit of FIG. 10, account is taken of the parallelresistance paths, and also in utilizing the circuit of FIG. 10, it isdesirable to keep the load on leads 208 and 212 small as possible, andto keep the terminating impedances as large as possible to reduce errorsin the indication circuit to a minimum.

Particularreference is made again to the circuit of FIG. 2. Aspreviously stated transistor 164 is normally cut off. It is renderedconductive by a bridge unbalance signal of the proper phase andmagnitude such as signal A" of FIG. 8C. When 164 becomes conductive, thepositive bias on the base of phase sensor transistor 133 is slightlydecreased to that a small decrease in signal on the base 132 does notallow the multivibrator to trip off. Until the circuit is reset by agreater decrease in signal, the multivibrator will remain on.

Particular reference is made now to FIG. 1 l, in which a modifiedcircuit is shown, according to an additional embodiment of theinvention. In FIG. 11 the bias control feedback link including rectifier150 and resistor 153, for controlling the bias on the phase sensingtransistor 133, is no longer employed. In FIG. 11, resistors 156 and 157joined bylead 158 and reset switch 185 interconnect the collector oftransistor 176 and the base of transistor 164. Lead 158 is connected byway of capacitor 159 to lead 111. Capacitor 159 serves as an integratorto prevent the circuit from tripping on at every single pulse. Thiseffectively prevents any random noise pulses from causing a false trip,but allows a repetitive signal pulse to correctly trip the circuit. Oncethe multivibrator has assumed a state in response to a signal pulse, itremains in that state until the circuit is opened at reset switch 185.

Again by way of summarizing the operation of the circuit of FIG. 2, oncethe multivibrator has tripped on, a certain length of time, which isdetermined by the value of resistor 166 and capacitor 168, must elapsebefore the circuit is restored for further operation. When the flip-flopreturns to its reset or original state, within one cycle, if the bridgeisstill above balance, it retrips. For a shut down signal, the firsttime the flip-flop trips is the only important occurrence. However, forturning on and off of heaters to control system pressure, the off pointis also important. It should be noted that in at least one embodimentshown themultivibrator circuit trips for a finite time, resets itself,and if told on the next cycle that it should still be tripped, it tripsagain within a cycle. Since the amount of time the flip-flop will staytripped can be adjusted, on-off control can be stabilized by varying thetime on instead of requiring the handling of two accurate signal levelpoints.

In all embodiments, resistance values and potential values may be easilyselected to provide for operation in the desired manner.

In FIG. 4, the R-C circuit 321-322 may be omitted if not needed. Aftertransistor 241 is momentarily turned off, for one alternation transistor242 may be on, with loss of voltage on lead 253. The R-C circuit insuresthat transistor 270 will not lose control during this brief period.

The term multivibrator amplifier" as employed herein and in the claimsappended hereto, includes a dual-state circuit which is in one stateuntil a signal causes it to change to a second state, and which remainsin the second state a substantial period of time determined at least inpart by the time constant of a time delay circuit included in theamplifier.

Whereas the invention has been shown and described with respect to someembodiments thereof which give staisfactory results, it should beunderstood that changes may be made and equivalents substituted withoutdeparting from the spirit and scope of the invention.

We claim as our invention:

1. Apparatus for providing from a single transducer responsive tovariations in avariable quantity an indication signal for indicatingchanges in the value of the variable quantity and a control signal whenvariations in the value of the variable quantity attain at least apredetermined magnitude, the indication and the control signals beingsubstantially independent of each other, comprising in combination,bridge means having a first branch consisting of a resistor in serieswith a trans ducer the resistance value of which varies in accordancewith variations in the variable quantity and a second branch comprisingmeans forming two parallel current paths, one of said path forming meansconsisting of a series-connected additional resistor and a potentiometerhaving an adjustable arm, the other of said path forming meansconsisting of a further resistor and a rheostat in series therewith,servo amplifier means con-' nected between the junction of thefirst-named resistor and the transducer and the junction between thefurther resistor and the rheostat to be energized by an unbalance signaldeveloped thereacross, servo motor means connected to the servoamplifier means to receive the output thereof, linkage meansconnectingthe servo motor means to the rheostat to vary the value of theresistance thereof in a direction which tends to reduce the unbalancesignal at the input of the servo amplifier means to zero, meansincluding a monostable multivibrator connected between the junction ofthe first-named resistor and the transducer and to the arm of thepotentiometer to be energized by the control signal developedthereacross whereby the multivibrator is substantially continually inone state while the control signal is present in a predetermined phaseand in at least a predetermined amplitude, and is in the other statewhile said control signal is not in said predetermined phaseirrespective of the amplitude thereof, and control means connected tothe multivibrator to be operated to one control condition when themultivibrator is in one state and to be operated to another controlcondition when the multivibrator is in the other state.

2. In a circuit for providing a null type indication of the instantvalue of a variable and for providing a control signal when the variabledeviates substantially from a predetermined value, in combination, meansforming a bridge circuit having two branches, one of said branchesincluding an element responsive to variations in the variable, the otherof said branches including means forming two resistance paths, meansconnected to the variable responsive element and to the means formingone of the resistance paths for obtaining a control signal, means foramplifying said control signal, controllable means connected to theamplifying means for utilizing the control signal, the means forming theother of said resistance paths including a variable resistor, circuitmeans connected to the variable responsive element and to the meansforming the other of the resistance paths and including a servoamplifier and a servo motor, and means connecting the servo motor tosaid variable resistor of the last-named path for adjusting the value ofsaid variable resistor automatically in accordance with variations inthe variable to thereby provide a null balance, and indicating meansconnected to the adjusting means for providing an indication inaccordance with the setting of the adjusting means.

3. In control circuit apparatus, in combination, a resistancethermometer, means forming an alternating current bridge, said bridgehaving the resistance thermometer connected in one branch thereof andbeing constructed and arranged whereby when the bridge is off balance inone sense a signal of one phase is developed at the output of the bridgeand when the bridge is off balance in the other sense a signal ofopposite phase is developed at the output of the bridge, amplifier meansconnected to the bridge for amplifying the output signal thereof, phasesensing means connected to the amplifier means, a monostable flip-flopcircuit connected to the phase sensing means to receive the outputtherefrom whereby the monostable flip-flop circuit is caused to changefrom a first state to a second state only when the bridge is unbalancedin at least a predetermined amount and in a predetermined sense, and acontrolled element operatively connected to the monostable flip-flopcircuit, the controlled element being changed from an on condition to anoff condition selectively in accordance with the state of the monostableflip-flop circuit.

4. In control apparatus, in combination, alternating current bridgemeans including a variable element responsive to variations in aquantity, means connected to the bridge means for amplifying the signaloutput thereof resulting from variations in the variable element, phasesensitive means including a transistor having a collector, an emitterand a base, circuit means connecting the phase sensitive means to theamplifying means to receive the output therefrom, means connected to thetransistor for applying a pulsating direct current potential to saidcollector of the same frequency as the alternating current, output meansconnected to the phase sensitive transistor means, the output meansdeveloping a pulsed signal thereacross when the input to the phasesensitive transistor means has a predetermined phase relationship withrespect to the pulsating direct current potential, a monostable flipflopcircuit including first and second transistors each having a base,collector, and emitter, the first transistor being connected to receiveas an input between the base and emitter thereof the pulsed signal ofthe output means, the pulsed signal of the output means causing themonostable circuit to switch between the two states thereof, andfeedback meansincluding a resistor and a rectifier connecting thecollector of the first transistor to the base of the transistor of thephase sensitive means, the feedback means providing means for varyingthe bias point of the transistor of the phase sensitive means.

5. Control circuit apparatus comprising, in combination, bridge meansincluding in one branch thereof a transducer having a resistance valuewhich varies in accordance with variations in a quantity, said bridgemeans including as energizing means an alternating current source ofpredetermined frequency, phase sensing transistor means including atransistor having a base, collector and emitter, means for applying ahalfwave rectified alternating current of the same frequency as that ofthe alternating current energizing the bridge means to said collector,means for amplifying the output of the bridge means and applying theamplified output as an input to the transistor of the phase sensingtransistor means between the base and emitter thereof, the phase sensingtransistor means supplying a pulse output only when the phase of theinput thereto has a predetermined relationship to the phase of therectified pulsating direct current applied to the collec tor thereof, amonostable flip-flop transistor circuit including first and secondtransistors each having a base, collector, and emitter, and positivefeedback circuit means connecting the collector of the second transistorto the base of the first transistor, the first transistor beingconnected to the phase sensing transistor to receive the pulse outputthereof, and output means including a controlled element connected tothe second transistor, said positive feedback circuit means includingmeans having an adjustable time constant, variations in said timeconstant varying the number of pulses in the output of the phase sensingtransistor means which elapse before the transition of the monostablecircuit between the two states thereof.

6. In a control circuit, in combination, a transducer having animpedance value which varies in accordance with variations in aquantity, bridge means having the transducer connected in one .branchthereof, the bridge means providing first and second signals when thebridge is unbalanced, a controlled device, a transistor monostablemultivibrator having a first state and a second state and having theoutput thereof operatively connected to the controlled device,thecontrolled device being in a first control condition while themultivibrator is in a first state and in a second control conditionwhile the multivibrator is in a second state, circuit means connectingthe multivibrator to the bridge means for utilizing the first signal tocause the multivibrator to change from a first state to a second statewhen the first signal has a pre-determined phase and at least apredetermined amplitude, and means including a normally unenergizeddevice connecting the bridge means to the multivibrator whereby thesecond signal is supplied to the multivibrator when the normallyunenergized device is energized, said unenergized device being connectedto the multivibrator whereby the device is energized when themultivibrator changes from a first state to a second state, the secondsignal when applied to the multivibrator in at least a predeterminedamplitude preventing the multivibrator from switching from the secondstate to the first state for a substantial period of time.

7. In control circuit apparatus, in combination, bridge circuit means,said bridge circuit means having connected therein transducer meansresponsive to variations in a variable quantity, said bridge circuitmeans providing first and second signals of first and second amplitudesrespectively for selectively turning on and off an element to becontrolled, signal amplifying means having the first signal appliedthereto, a monostable'transistor multivibrator having the amplifiedfirst signal applied thereto and being operatively connected to theelement to be controlled whereby the element to be controlled is onwhile'the multivibrator is in one state and the element to be controlledis off while the multivibrator is in the other state, the amplifiedfirst signal causing the multivibrator to undergo a transition between afirst state and a second state, a transistor operatively connected tothe multivibrator to have an energizing potential applied thereto whilethe multivibrator is in the second state and-having no energizingpotential applied thereto while the multivibrator is in the first state,circuit means connecting the transistor to the bridge circuit means tohave the second signal applied thereto, and other circuit meansconnecting the transistor to the multivibrator whereby when energizingpotential is applied to the transistor the second signal, afteramplification by the transistor, is applied to the multivibrator andwhen of at least a predetermined amplitude prevents the multivibratorfrom changing from the second state to the first state for a substantialperiod of time.

8. In control circuit apparatus, in combination, a source of alternatingcurrent potential, a transducer responsive to variations in a variable,bridge means connected to the source and having the transducer connectedin circuit therein for providing an output signal at unbalance, theoutput signal varying in phase in accordance with the sense of unbalanceof the bridge means, means for amplifying the output signal, atransistor having a base, emitter and collector, means for applying theamplified signal between said base and emitter, means connected to thesource of alternating current potential for obtaining a half-waverectified voltage and for applying the half-wave rectified voltage tosaid collector, said transistor supplying output pulses only when theinput thereto has a predetermined phase relationship with the half-waverectified voltage on the collector thereof, and a monostable flip-floptransistor circuit connected to said collector to receive pulsestherefrom, said pulses when of at least a predetermined amplitudecausing the monostable flip-flop circuit to change from a first state toa second state.

9. Control apparatus comprising, in combination, a differentialtransformer including a primary, first and second secondaries and amovable slug core of magnetizable material, circuit means including asource of alternating current potential for energizing the primary,

means connected to the slug core whereby the Slug core has the position'thereof varied in accordance with variations in a quantity, bridgecircuit means connecting the first and second secondaries in series toform one branch of the bridge, the other branch of the bridge circuitmeans including in series a resistor and a potentiometer having anadjustable arm, output means connected to the arm and the junctionbetween the first and second secondaries, means for amplifying thesignal developed at the output means, phase sensing means operativelyconnected to the amplifying means and to the source of alternatingcurrent potential, a multivibrator. connected to the phase sensing meansto be switched between states when the input to the phase sensing meanshas a predetermined phase and at least a predetermined amplitude, andutilization means including a controlled device connected to themultivibrator.

10. In a circuit for providing a null type indication of the instantvalue of variable quantity and for providing a control signal when thevariable-deviates by a predetermined amount from a predetermined value,in combination, a resistance thermometer having a resistance value whichvaries with the variable quantity, bridge means having the resistancethermometer in one branch thereof, variable resistance means in theother branch thereof, a source of alternating current potentialconnected to the bridge means, circuit means connected to the bridgemeans for obtaining an alternating current control signal and analternating current indication signal, servo means having the indicationsignal applied thereto, the servo means being operatively connected tothe variable resistance means and utilizing the indication signal tovary theresistance in a manner which tends to reduce the indicationerror signal to zero, a phase sensor device connected to the source ofalternating current potential and to the bridge means to have thecontrol signal applied thereto, the phase sensor device providing anoutput only when the control signal has a predetermined phaserelationship to the alternating current, multivibrator means connectedto the phase sensor device and being switched between states when theoutput of the phase sensor device has at least a predeterminedamplitude, and utilization means connected to the multivibrator means.

11. In indication and control apparatus for obtaining first and secondcontrol signals and an indication signal, in combination, a source ofalternating current potential, a transducer having an impedance whichvaries with variations in a quantity, and a bridge connected to thesource of alternating current potential to be energized therefrom, thebridge having the transducer in one branch thereof, the other branch ofthe bridge including means forming first, second, and third resistancepaths all connected in parallel, the first path having a firstpotentiometer therein having an adjustable arm, the first control signalbeing obtained from said arm, the second path having a secondpotentiometer therein having an adjustable arm, the second controlsignal being obtained from said last-named arm, the third path having avariable resistor therein adapted to have the resistance value thereofvaried to rebalance the third path of the branch of the bridge after theimpedance of the transducer changes.

12. In indication and control apparatus for obtaining a control signaland an indication signal, in combination, a source of alternatingcurrent potential, a transducer having an impedance which varies withvariations in a quantity, a bridge connected to the source ofalternating current potential to be energized therefrom, the bridgehaving the transducer in one brahch thereof, the other branch of thebridge including means forming first and second resistance pathsconnected in parallel, the first path having a potentiometer thereinhaving an adjustable arm, the control signal being obtained from saidarm, the second path having a variable resistor therein adapted to havethe resistance value thereof varied to rebalance the second path of thebridge after the impedance of the transducer changes.

13. In a circuit for providing a null type indication of the instantvalue of a variable quantity and for providing a control signal when thevariable deviates by a predetermined amount from a predetermined value,in combination, a differential transformer including a primary, firstand second secondaries and a movable slug core composed of magnetizablematerial, circuit means including a source of alternating currentpotential for energizing the primary, means connected to the slug corewhereby the slug core has the position thereof varied in accordance withvariations in the quantity, bridge circuit means connecting the firstand second secondaries in series to form one branch of the bridge,variable resistance means in' the other branch thereof, circuit meansconnected to the bridge means for obtaining a control signal and anindication signal, servo means having the indication signal appliedthereto, the servo means being operatively connected to the variableresistance means and utilizing the indication signal to vary theresistance in a manner which tends to reduce the indication signal tozero, multivibrator means, further circuit means connecting themultivibrator means to the bridge means for utilizing the control signalto switch the multivibrator means between states, and utilization meansconnected to the multivibrator means.

14. In a control circuit of the type in which the instant amplitude of avariable alternating current signal corresponds to the instant deviationof a quantity to be controlled from a preselected value and the phase ofthe signal reverses as the amplitude of the signal goes through zero asthe signal changes in sense from a value which deviates in one directionfrom the preselected value to one which deviates in the other directionfrom the preselected value, and in which the signal is utilized to turnon and off a device which while on varies the quantity in a manner torestore the quantity to the preselected value, in combination,alternating current signal obtaining means constructed and arranged toprovide a variable signal which varies in amplitude in accordance withvariations in the quantity from the preselected value and reverses inphase when the sense of the variation changes, phase sensing transistormeans having the signal applied thereto and including a transistornormally biased slightly beyond saturation and having a pulsating directcurrent potential of the same frequency as the alternating currentsignal applied thereto for providing an output when the signal has atleast a predetermined amplitude and a predetermined phase relationshipto said pulsating direct current potential, a multivibrator connected tothe phase sensing transistor means and being switched from a firstnormal state to a second state by the output of the phase sensingtransistor means, the multivibrator including time constant means formaintaining the multivibrator in the second state a predetermined timeinterval after which the multivibrator returns to the first state, meansadapted to connect the multivibrator to the device, and meansoperatively connected to the multivibrator and to the phase sensingtransistor means for varying the bias on the transistor of the phasesensing transistor means toward saturation when the multivibratorswitches to the second state whereby a signal at the transistor meanssmaller in amplitude relative to said predetermined amplitude willthereafter cause the multivibrator to switch from the first state to thesecond state.

15. In control apparatus, in combination, alternating current bridgemeans, the alternating current bridge means including a source ofalternating current potential, phase sensing means operatively connectedto the bridge means and having the output signal of the bridge meansapplied thereto, circuit means connecting the phase sensing means to thealternating current source, the phase sensing means including atransistor normally biased beyond saturation, the circuit meansconnecting the phase sensing means to the alternating current sourceincluding half-wave rectifier means for obtaining a pulsating directcurrent potential of predetermined polarity and applying the pulsesthereof to the collector of the transistor to energize the same, thetransistor providing no output when the phase of the signal appliedthereto is of a polarity to increase the bias while a pulse is appliedto said collector, the phase sensing means supplying an output signalwhen the signal applied thereto reduces the bias coincidentally with theappearance of the pulse on the collector thereof, multivibrator meansincludinga time delay circuit operatively connected to the phase sensingmeans, the multivibrator means being switched from a first state to asecond state by the application of an input signal thereto from thephase sensing means, the multivibrator means thereafter returning fromthe second state to the first state after a period of time determined bythe time constant of the time delay circuit, the appearance thereafterof a signal in the output of the phase sensing means causing themultivibrator to again change from the first state to the second state.

16. In control apparatus, in combination, means including alternatingcurrent bridge means and a biased amplifier, the biased amplifier beingnormally biased at a point whereby a signal of at least a firstpredetermined amplitude causes an output from the biased amplifier, amultivibrator circuit connected to the biased amplifier to receive theoutput signal therefrom, the output signal causing the multivibratorcircuit to change from a first stage to a second state, themultivibrator circuit including time delay means for returning themultivibrator circuit to the first state after the lapse of apredetermined time, and circuit means connecting the multivibratorcircuit to the biased amplifier for altering the bias on the biasedamplifier when the multivibrator circuit assumes the second statewhereby a sig nal of a second predetermined amplitude less than saidfirst predetermined amplitude produces an output sufficient to cause themultivibrator to change from the first state to the second state.

17. In control apparatus, in combination, alternating current bridgemeans having in one branch thereof means for obtaining first and secondsignals of two different amplitudes, a first amplitude and a secondamplitude respectively, a multivibrator circuit having a first state anda second state, circuit means connecting the bridge means to themultivibrator circuit for utilizing the first signal of the firstamplitude to cause the multivibrator to switch from the first state tothe second state, normally deenergized other circuit means con nectingthe bridge means to the multivibrator circuit, means connecting theother circuit means to the multivibrator circuit whereby the othercircuit means is energized when the multivibrator circuit changes to thesecond state, the other circuit means thereupon providing that a secondsignal of said second amplitude is thereafter sufficient to cause themultivibrator circuit to change from the first state to the secondstate.

18. In control and indicating apparatus, in combination, alternatingcurrent bridge means including transducer means for supplying at leasttwo output signals from first and second branches, servo means forutiliz 1 ing the first output signal to maintain the first branch of thebridge in a balanced condition and provide an indication of the instantvalue of a variable quantity in terms of the instant condition of saidtransducer means in the bridge, a controlled device, and circuit meansconnecting the controlled device to the second branch of the bridgemeans for utilizing the second signal to switch the controlled devicebetween a first control condition and a second control condition, thelastnamed circuit means including means for maintaining the controlleddevice in a condition to change the variable quantity toward the normaldesired value thereof while the second signal is changing in amplitude.

19. In control and indication apparatus, in combination, alternatingcurrent bridge means including a first branch and a second branch, boththe first branch and the second branch including the same transducermeans responsive to variations in a variable quantity, the first andsecond branches including means for obtaining at least two signals whichvary with variations in said quantity, servo means connected to thebridge for utilizing the first signal to maintain the first branch ofthe bridge in a balanced condition, indicator means operativelyconnected to the servo means for giving an indication of the quantity interms of the output of the transducer means, a controlled device,circuit means including a multivibrator connecting the controlled deviceto the second branch of the bridge means, the second signal causing themultivibrator to switch from a first condition to a second condition atwhich second condition the controlled device is operated and thevariable quantity tends to be changed toward a normal value, the circuitmeans including means for maintaining the controlled device operatedwhile the signal from the transducer means is falling in amplitude asthe variable quantity varies in magnitude toward said normal condition.

20. In control apparatus, in combination, a controlled device which ismoved to a first control condition when the value of a variable quantityreaches at least a predetermined value and which is moved to a secondcontrol condition when the variable quantity thereafter changes to asecond predetermined value, alternating current bridge means responsiveto variations in the quantity for obtaining first and second signals ofdifferent amplitudes, switching means including amplifier means adaptedto change the controlled device from the first control condition to thesecond control condition, first and second parallel circuit pathsconnecting the switching means to the bridge means and having the firstsignal applied to the first path and the second signal applied to thesecond path, the second path including normally deenergized means forpreventing the passage of the signal therethrough, and means connectingthe amplifier means to the normally deenergized means for energizing thelast-named means and activating the second circuit path after the firstcircuit path has caused the controlled device to move from one controlposition to the other control position, the signal through the secondpath causing the control device to remain in the other control conditionuntil the second signal changes to at least a predetermined thirdamplitude.

1. Apparatus for providing from a single transducer responsive tovariations in a variable quantity an indication signal for indicatingchanges in the value of the variable quantity and a control signal whenvariations in the value of the variable quantity attain at least apredetermined magnitude, the indication and the control signals beingsubstantially independent of each other, comprising in combination,bridge means having a first branch consisting of a resistor in serieswith a transducer the resistance value of which varies in accordancewith variations in the variable quantity and a second branch comprisingmeans forming two parallel current paths, one of said path forming meansconsisting of a series-connected additional resistor and a potentiometerhaving an adjustable arm, the other of said path forming meansconsisting of a further resistor and a rheostat in series therewith,servo amplifier means connected between the junction of the first-namedresistor and the transducer and the junction between the furtherresistor and the rheostat to be energized by an unbalance signaldeveloped thereacross, servo motor means connected to the servoamplifier means to receive the output thereof, linkage means connectingthe servo motor means to the rheostat to vary the value of theresistance thereof in a direction which tends to reduce the unbalancesignal at the input of the servo amplifier means to zero, meansincluding a monostable multivibrator connected between the junction ofthe first-named resistor and the transducer and to the arm of thepotentiometer to be energized by the control signal developedthereacross whereby the multivibrator is substantially continually inone state while the control signal is present in a predetermined phaseand in at least a predetermined amplitude, and is in the other statewhile said control signal is not in said predetermined phaseirrespective of the amplitude thereof, and control means connected tothe multivibrator to be operated to one control condition when themultivibrator is in one state and to be operaTed to another controlcondition when the multivibrator is in the other state.
 2. In a circuitfor providing a null type indication of the instant value of a variableand for providing a control signal when the variable deviatessubstantially from a predetermined value, in combination, means forminga bridge circuit having two branches, one of said branches including anelement responsive to variations in the variable, the other of saidbranches including means forming two resistance paths, means connectedto the variable responsive element and to the means forming one of theresistance paths for obtaining a control signal, means for amplifyingsaid control signal, controllable means connected to the amplifyingmeans for utilizing the control signal, the means forming the other ofsaid resistance paths including a variable resistor, circuit meansconnected to the variable responsive element and to the means formingthe other of the resistance paths and including a servo amplifier and aservo motor, and means connecting the servo motor to said variableresistor of the last-named path for adjusting the value of said variableresistor automatically in accordance with variations in the variable tothereby provide a null balance, and indicating means connected to theadjusting means for providing an indication in accordance with thesetting of the adjusting means.
 3. In control circuit apparatus, incombination, a resistance thermometer, means forming an alternatingcurrent bridge, said bridge having the resistance thermometer connectedin one branch thereof and being constructed and arranged whereby whenthe bridge is off balance in one sense a signal of one phase isdeveloped at the output of the bridge and when the bridge is off balancein the other sense a signal of opposite phase is developed at the outputof the bridge, amplifier means connected to the bridge for amplifyingthe output signal thereof, phase sensing means connected to theamplifier means, a monostable flip-flop circuit connected to the phasesensing means to receive the output therefrom whereby the monostableflip-flop circuit is caused to change from a first state to a secondstate only when the bridge is unbalanced in at least a predeterminedamount and in a predetermined sense, and a controlled elementoperatively connected to the monostable flip-flop circuit, thecontrolled element being changed from an on condition to an offcondition selectively in accordance with the state of the monostableflip-flop circuit.
 4. In control apparatus, in combination, alternatingcurrent bridge means including a variable element responsive tovariations in a quantity, means connected to the bridge means foramplifying the signal output thereof resulting from variations in thevariable element, phase sensitive means including a transistor having acollector, an emitter and a base, circuit means connecting the phasesensitive means to the amplifying means to receive the output therefrom,means connected to the transistor for applying a pulsating directcurrent potential to said collector of the same frequency as thealternating current, output means connected to the phase sensitivetransistor means, the output means developing a pulsed signalthereacross when the input to the phase sensitive transistor means has apredetermined phase relationship with respect to the pulsating directcurrent potential, a monostable flip-flop circuit including first andsecond transistors each having a base, collector, and emitter, the firsttransistor being connected to receive as an input between the base andemitter thereof the pulsed signal of the output means, the pulsed signalof the output means causing the monostable circuit to switch between thetwo states thereof, and feedback means including a resistor and arectifier connecting the collector of the first transistor to the baseof the transistor of the phase sensitive means, the feedback meansproviding means for varying the bias point of the transistor of thephase sensitive meAns.
 5. Control circuit apparatus comprising, incombination, bridge means including in one branch thereof a transducerhaving a resistance value which varies in accordance with variations ina quantity, said bridge means including as energizing means analternating current source of predetermined frequency, phase sensingtransistor means including a transistor having a base, collector andemitter, means for applying a half-wave rectified alternating current ofthe same frequency as that of the alternating current energizing thebridge means to said collector, means for amplifying the output of thebridge means and applying the amplified output as an input to thetransistor of the phase sensing transistor means between the base andemitter thereof, the phase sensing transistor means supplying a pulseoutput only when the phase of the input thereto has a predeterminedrelationship to the phase of the rectified pulsating direct currentapplied to the collector thereof, a monostable flip-flop transistorcircuit including first and second transistors each having a base,collector, and emitter, and positive feedback circuit means connectingthe collector of the second transistor to the base of the firsttransistor, the first transistor being connected to the phase sensingtransistor to receive the pulse output thereof, and output meansincluding a controlled element connected to the second transistor, saidpositive feedback circuit means including means having an adjustabletime constant, variations in said time constant varying the number ofpulses in the output of the phase sensing transistor means which elapsebefore the transition of the monostable circuit between the two statesthereof.
 6. In a control circuit, in combination, a transducer having animpedance value which varies in accordance with variations in aquantity, bridge means having the transducer connected in one branchthereof, the bridge means providing first and second signals when thebridge is unbalanced, a controlled device, a transistor monostablemultivibrator having a first state and a second state and having theoutput thereof operatively connected to the controlled device, thecontrolled device being in a first control condition while themultivibrator is in a first state and in a second control conditionwhile the multivibrator is in a second state, circuit means connectingthe multivibrator to the bridge means for utilizing the first signal tocause the multivibrator to change from a first state to a second statewhen the first signal has a pre-determined phase and at least apredetermined amplitude, and means including a normally unenergizeddevice connecting the bridge means to the multivibrator whereby thesecond signal is supplied to the multivibrator when the normallyunenergized device is energized, said unenergized device being connectedto the multivibrator whereby the device is energized when themultivibrator changes from a first state to a second state, the secondsignal when applied to the multivibrator in at least a predeterminedamplitude preventing the multivibrator from switching from the secondstate to the first state for a substantial period of time.
 7. In controlcircuit apparatus, in combination, bridge circuit means, said bridgecircuit means having connected therein transducer means responsive tovariations in a variable quantity, said bridge circuit means providingfirst and second signals of first and second amplitudes respectively forselectively turning on and off an element to be controlled, signalamplifying means having the first signal applied thereto, a monostabletransistor multivibrator having the amplified first signal appliedthereto and being operatively connected to the element to be controlledwhereby the element to be controlled is on while the multivibrator is inone state and the element to be controlled is off while themultivibrator is in the other state, the amplified first signal causingthe multivibrator to undergo a transition between a first stAte and asecond state, a transistor operatively connected to the multivibrator tohave an energizing potential applied thereto while the multivibrator isin the second state and having no energizing potential applied theretowhile the multivibrator is in the first state, circuit means connectingthe transistor to the bridge circuit means to have the second signalapplied thereto, and other circuit means connecting the transistor tothe multivibrator whereby when energizing potential is applied to thetransistor the second signal, after amplification by the transistor, isapplied to the multivibrator and when of at least a predeterminedamplitude prevents the multivibrator from changing from the second stateto the first state for a substantial period of time.
 8. In controlcircuit apparatus, in combination, a source of alternating currentpotential, a transducer responsive to variations in a variable, bridgemeans connected to the source and having the transducer connected incircuit therein for providing an output signal at unbalance, the outputsignal varying in phase in accordance with the sense of unbalance of thebridge means, means for amplifying the output signal, a transistorhaving a base, emitter and collector, means for applying the amplifiedsignal between said base and emitter, means connected to the source ofalternating current potential for obtaining a half-wave rectifiedvoltage and for applying the half-wave rectified voltage to saidcollector, said transistor supplying output pulses only when the inputthereto has a predetermined phase relationship with the half-waverectified voltage on the collector thereof, and a monostable flip-floptransistor circuit connected to said collector to receive pulsestherefrom, said pulses when of at least a predetermined amplitudecausing the monostable flip-flop circuit to change from a first state toa second state.
 9. Control apparatus comprising, in combination, adifferential transformer including a primary, first and secondsecondaries and a movable slug core of magnetizable material, circuitmeans including a source of alternating current potential for energizingthe primary, means connected to the slug core whereby the slug core hasthe position thereof varied in accordance with variations in a quantity,bridge circuit means connecting the first and second secondaries inseries to form one branch of the bridge, the other branch of the bridgecircuit means including in series a resistor and a potentiometer havingan adjustable arm, output means connected to the arm and the junctionbetween the first and second secondaries, means for amplifying thesignal developed at the output means, phase sensing means operativelyconnected to the amplifying means and to the source of alternatingcurrent potential, a multivibrator connected to the phase sensing meansto be switched between states when the input to the phase sensing meanshas a predetermined phase and at least a predetermined amplitude, andutilization means including a controlled device connected to themultivibrator.
 10. In a circuit for providing a null type indication ofthe instant value of variable quantity and for providing a controlsignal when the variable deviates by a predetermined amount from apredetermined value, in combination, a resistance thermometer having aresistance value which varies with the variable quantity, bridge meanshaving the resistance thermometer in one branch thereof, variableresistance means in the other branch thereof, a source of alternatingcurrent potential connected to the bridge means, circuit means connectedto the bridge means for obtaining an alternating current control signaland an alternating current indication signal, servo means having theindication signal applied thereto, the servo means being operativelyconnected to the variable resistance means and utilizing the indicationsignal to vary the resistance in a manner which tends to reduce theindication error signal to zero, a phase sensor device connected to Thesource of alternating current potential and to the bridge means to havethe control signal applied thereto, the phase sensor device providing anoutput only when the control signal has a predetermined phaserelationship to the alternating current, multivibrator means connectedto the phase sensor device and being switched between states when theoutput of the phase sensor device has at least a predeterminedamplitude, and utilization means connected to the multivibrator means.11. In indication and control apparatus for obtaining first and secondcontrol signals and an indication signal, in combination, a source ofalternating current potential, a transducer having an impedance whichvaries with variations in a quantity, and a bridge connected to thesource of alternating current potential to be energized therefrom, thebridge having the transducer in one branch thereof, the other branch ofthe bridge including means forming first, second, and third resistancepaths all connected in parallel, the first path having a firstpotentiometer therein having an adjustable arm, the first control signalbeing obtained from said arm, the second path having a secondpotentiometer therein having an adjustable arm, the second controlsignal being obtained from said last-named arm, the third path having avariable resistor therein adapted to have the resistance value thereofvaried to rebalance the third path of the branch of the bridge after theimpedance of the transducer changes.
 12. In indication and controlapparatus for obtaining a control signal and an indication signal, incombination, a source of alternating current potential, a transducerhaving an impedance which varies with variations in a quantity, a bridgeconnected to the source of alternating current potential to be energizedtherefrom, the bridge having the transducer in one branch thereof, theother branch of the bridge including means forming first and secondresistance paths connected in parallel, the first path having apotentiometer therein having an adjustable arm, the control signal beingobtained from said arm, the second path having a variable resistortherein adapted to have the resistance value thereof varied to rebalancethe second path of the bridge after the impedance of the transducerchanges.
 13. In a circuit for providing a null type indication of theinstant value of a variable quantity and for providing a control signalwhen the variable deviates by a predetermined amount from apredetermined value, in combination, a differential transformerincluding a primary, first and second secondaries and a movable slugcore composed of magnetizable material, circuit means including a sourceof alternating current potential for energizing the primary, meansconnected to the slug core whereby the slug core has the positionthereof varied in accordance with variations in the quantity, bridgecircuit means connecting the first and second secondaries in series toform one branch of the bridge, variable resistance means in the otherbranch thereof, circuit means connected to the bridge means forobtaining a control signal and an indication signal, servo means havingthe indication signal applied thereto, the servo means being operativelyconnected to the variable resistance means and utilizing the indicationsignal to vary the resistance in a manner which tends to reduce theindication signal to zero, multivibrator means, further circuit meansconnecting the multivibrator means to the bridge means for utilizing thecontrol signal to switch the multivibrator means between states, andutilization means connected to the multivibrator means.
 14. In a controlcircuit of the type in which the instant amplitude of a variablealternating current signal corresponds to the instant deviation of aquantity to be controlled from a preselected value and the phase of thesignal reverses as the amplitude of the signal goes through zero as thesignal changes in sense from a value which deviates in one directionfrom the Preselected value to one which deviates in the other directionfrom the preselected value, and in which the signal is utilized to turnon and off a device which while on varies the quantity in a manner torestore the quantity to the preselected value, in combination,alternating current signal obtaining means constructed and arranged toprovide a variable signal which varies in amplitude in accordance withvariations in the quantity from the preselected value and reverses inphase when the sense of the variation changes, phase sensing transistormeans having the signal applied thereto and including a transistornormally biased slightly beyond saturation and having a pulsating directcurrent potential of the same frequency as the alternating currentsignal applied thereto for providing an output when the signal has atleast a predetermined amplitude and a predetermined phase relationshipto said pulsating direct current potential, a multivibrator connected tothe phase sensing transistor means and being switched from a firstnormal state to a second state by the output of the phase sensingtransistor means, the multivibrator including time constant means formaintaining the multivibrator in the second state a predetermined timeinterval after which the multivibrator returns to the first state, meansadapted to connect the multivibrator to the device, and meansoperatively connected to the multivibrator and to the phase sensingtransistor means for varying the bias on the transistor of the phasesensing transistor means toward saturation when the multivibratorswitches to the second state whereby a signal at the transistor meanssmaller in amplitude relative to said predetermined amplitude willthereafter cause the multivibrator to switch from the first state to thesecond state.
 15. In control apparatus, in combination, alternatingcurrent bridge means, the alternating current bridge means including asource of alternating current potential, phase sensing means operativelyconnected to the bridge means and having the output signal of the bridgemeans applied thereto, circuit means connecting the phase sensing meansto the alternating current source, the phase sensing means including atransistor normally biased beyond saturation, the circuit meansconnecting the phase sensing means to the alternating current sourceincluding half-wave rectifier means for obtaining a pulsating directcurrent potential of predetermined polarity and applying the pulsesthereof to the collector of the transistor to energize the same, thetransistor providing no output when the phase of the signal appliedthereto is of a polarity to increase the bias while a pulse is appliedto said collector, the phase sensing means supplying an output signalwhen the signal applied thereto reduces the bias coincidentally with theappearance of the pulse on the collector thereof, multivibrator meansincluding a time delay circuit operatively connected to the phasesensing means, the multivibrator means being switched from a first stateto a second state by the application of an input signal thereto from thephase sensing means, the multivibrator means thereafter returning fromthe second state to the first state after a period of time determined bythe time constant of the time delay circuit, the appearance thereafterof a signal in the output of the phase sensing means causing themultivibrator to again change from the first state to the second state.16. In control apparatus, in combination, means including alternatingcurrent bridge means and a biased amplifier, the biased amplifier beingnormally biased at a point whereby a signal of at least a firstpredetermined amplitude causes an output from the biased amplifier, amultivibrator circuit connected to the biased amplifier to receive theoutput signal therefrom, the output signal causing the multivibratorcircuit to change from a first stage to a second state, themultivibrator circuit including time delay means for returning themultivibrator cIrcuit to the first state after the lapse of apredetermined time, and circuit means connecting the multivibratorcircuit to the biased amplifier for altering the bias on the biasedamplifier when the multivibrator circuit assumes the second statewhereby a signal of a second predetermined amplitude less than saidfirst predetermined amplitude produces an output sufficient to cause themultivibrator to change from the first state to the second state.
 17. Incontrol apparatus, in combination, alternating current bridge meanshaving in one branch thereof means for obtaining first and secondsignals of two different amplitudes, a first amplitude and a secondamplitude respectively, a multivibrator circuit having a first state anda second state, circuit means connecting the bridge means to themultivibrator circuit for utilizing the first signal of the firstamplitude to cause the multivibrator to switch from the first state tothe second state, normally deenergized other circuit means connectingthe bridge means to the multivibrator circuit, means connecting theother circuit means to the multivibrator circuit whereby the othercircuit means is energized when the multivibrator circuit changes to thesecond state, the other circuit means thereupon providing that a secondsignal of said second amplitude is thereafter sufficient to cause themultivibrator circuit to change from the first state to the secondstate.
 18. In control and indicating apparatus, in combination,alternating current bridge means including transducer means forsupplying at least two output signals from first and second branches,servo means for utilizing the first output signal to maintain the firstbranch of the bridge in a balanced condition and provide an indicationof the instant value of a variable quantity in terms of the instantcondition of said transducer means in the bridge, a controlled device,and circuit means connecting the controlled device to the second branchof the bridge means for utilizing the second signal to switch thecontrolled device between a first control condition and a second controlcondition, the last-named circuit means including means for maintainingthe controlled device in a condition to change the variable quantitytoward the normal desired value thereof while the second signal ischanging in amplitude.
 19. In control and indication apparatus, incombination, alternating current bridge means including a first branchand a second branch, both the first branch and the second branchincluding the same transducer means responsive to variations in avariable quantity, the first and second branches including means forobtaining at least two signals which vary with variations in saidquantity, servo means connected to the bridge for utilizing the firstsignal to maintain the first branch of the bridge in a balancedcondition, indicator means operatively connected to the servo means forgiving an indication of the quantity in terms of the output of thetransducer means, a controlled device, circuit means including amultivibrator connecting the controlled device to the second branch ofthe bridge means, the second signal causing the multivibrator to switchfrom a first condition to a second condition at which second conditionthe controlled device is operated and the variable quantity tends to bechanged toward a normal value, the circuit means including means formaintaining the controlled device operated while the signal from thetransducer means is falling in amplitude as the variable quantity variesin magnitude toward said normal condition.
 20. In control apparatus, incombination, a controlled device which is moved to a first controlcondition when the value of a variable quantity reaches at least apredetermined value and which is moved to a second control conditionwhen the variable quantity thereafter changes to a second predeterminedvalue, alternating current bridge means responsive to variations in thequantity for obtaining first and second signals of different amplitudes,switching means including amplifier means adapted to change thecontrolled device from the first control condition to the second controlcondition, first and second parallel circuit paths connecting theswitching means to the bridge means and having the first signal appliedto the first path and the second signal applied to the second path, thesecond path including normally deenergized means for preventing thepassage of the signal therethrough, and means connecting the amplifiermeans to the normally deenergized means for energizing the last-namedmeans and activating the second circuit path after the first circuitpath has caused the controlled device to move from one control positionto the other control position, the signal through the second pathcausing the control device to remain in the other control conditionuntil the second signal changes to at least a predetermined thirdamplitude.